1. Field of the Invention
The invention relates to methods and systems for controlling distortion of sound emitted by a speaker. In typical embodiments, distortion is controlled by limiting distortion in a critical frequency band (of the audio signal being reproduced) in which total harmonic distortion is likely to occur, using control parameters that have been predetermined in a speaker measurement operation.
2. Background of the Invention
Throughout this disclosure including in the claims, the expression performing an operation (e.g., filtering or compression) “on” signals or data is used in a broad sense to denote performing the operation directly on the signals or data, or on processed versions of the signals or data (e.g., on versions of the signals that have undergone preliminary filtering prior to performance of the operation thereon). Similarly, the expression “applying a filter to” a signal is used in a broad sense to denote applying the filter directly to the signal, or to a processed version of the signal (e.g., on a version of the signal that has undergone preliminary attenuation or other filtering prior to application of the relevant filter thereto).
Throughout this disclosure including in the claims, the expression “system” is used in a broad sense to denote a device, system, or subsystem. For example, a subsystem that determines a filter may be referred to as a filter system, and a system including such a subsystem (e.g., a system including a loudspeaker and means for applying the filter in the loudspeaker's signal path, as well as the subsystem that determines the filter) may also be referred to as a filter system.
Throughout this disclosure including in the claims, the terms “speaker” and “loudspeaker” are used synonymously to denote any sound-emitting transducer.
Throughout this disclosure including in the claims, the expression “reproduction” of a signal (e.g., an “input” signal or an “input audio” signal) by a speaker denotes causing the speaker to produce sound in response to the signal, including by performing any required amplification and/or other processing of the signal.
Mobile phone speakers and other miniature speakers often exhibit large amounts of distortion which limit the maximum volume at which they can play back audio in a pleasant manner for the user. The output of any speaker is subject to distortion, e.g., when the speaker is driven excessively at frequencies in a range in which total harmonic distortion (THD) peaks (i.e., exhibits a local maximum).
Until the present invention it had not been known how to limit distortion of the output of a speaker (e.g., a miniature speaker) in a computationally efficient manner by limiting distortion in a critical frequency band (of the audio signal being reproduced) in which THD peaks, using control parameters that have been predetermined in a speaker measurement operation.
Throughout this disclosure including in the claims, each of the expressions “total harmonic distortion” and “THD” (of a signal) is used in a broad sense to denote the total harmonic distortion of the signal (in the sense well known in the art) or any value (e.g., any measured value) indicative of an estimate of the signal's total harmonic distortion. An example of a signal's “THD” (as used herein) is a measurement indicative of THD plus noise (THD+N) of the signal.
It has been proposed to employ other techniques (e.g., Volterra-Wiener modeling) to limit speaker distortion but those techniques typically demand a high level of computational complexity and thus cannot be implemented in a practical manner on portable sound devices such as cell phones or in other applications that require a high degree of computational efficiency.
Typical embodiments of the inventive distortion control system are useful in combination with a compressor and/or a limiter to increase the average (e.g., RMS) power of the output of a speaker while preventing digital clipping. A compressor limits overall dynamic range of an audio signal to be reproduced, typically by detecting signal level (averaged over some time) and amplifying the signal by a time-varying amount whose instantaneous value is determined by the corresponding detected level. One type of compressor, known as an RMS compressor, detects RMS power of the signal and amplifies the signal by a time-varying amount whose instantaneous value is determined by the corresponding detected RMS power. A limiter scans for peaks in an audio signal to be reproduced, and attenuates the signal at (or around) each peak, typically in a manner that prevents clipping. A “compressor/limiter” includes a compressor stage in sequence with a limiter stage (typically, the limiter stage follows the compressor stage in the signal processing chain).
The use of a compressor or compressor/limiter can ensure sure that even quiet content (which does not utilize the digital PCM domain optimally) sounds loud and clear on a mobile phone speaker (or other speaker). For already loud content (that is, content which utilizes the digital PCM domain optimally, such as some recently mixed pop music), the compressor (or compressor/limiter) should not modify the audio significantly so as not to cause undue digital distortion.
Herein, we use the expression “loudness maximizer” to denote a system, typically including a compressor and/or a limiter, configured to limit the dynamic range of a reproduced audio signal and to increase the perceived overall average loudness level of the reproduced audio signal, preferably without introducing audible undesirable processing artifacts.
Until the present invention, it had not been known how to implement a loudness maximizer to limit distortion of the output of a speaker (e.g., a miniature speaker) in a computationally efficient manner by limiting distortion in a critical frequency band (of the audio signal being reproduced) in which THD peaks, using control parameters that have been predetermined in a speaker measurement operation. In a class of embodiments, the present invention is a loudness maximizer which includes an embodiment of the inventive distortion control system, and optionally also at least one of a compressor (e.g., an RMS compressor), a speaker equalization (EQ) correction module, a bass boost module, and a limiter. If present, the limiter is typically implemented as the final stage in the signal processing chain.